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    Influence of Preload in Flexibility Testing of Native and Instrumented Lumbar Spine Specimens

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    In-vitro investigations on spine specimens are usually performed to evaluate the biomechanical behavior of the native spine, or to determine the stabilizing effects of different types of implants. These investigations are currently performed by the application of pure moments in the anatomic directions. The inclusion of an additional preload is heavily discussed, but seldom used in the experiments. Investigators often mention that different preloads have an influence on the results, but a detailed analysis of the influence on specific outcome parameters is still missing. The aim of this study was to show the effects of preload on the load-displacement behavior of spine specimens and to give basic data upon which investigators of future studies may decide on the necessity of its inclusion, depending on their specific question. A six degree-of-freedom loading device has been developed and was used to load the specimens. It has a parallel architecture and is equipped with a hybrid position/force controller. Sixteen human lumbar spine specimens L1-L3 were investigated. The spines were tested native and in two stabilized corpectomy situations. An anterior stabilization with VentroFix and SynEx was used with and without posterior USS fixation. Compressive preloads of ON - 400N were applied and the flexibility tests carried out. The orientation of the preload was changing in accordance to the movement of the specimen.

    A general reduction of the range of motion (ROM) of up to 40% due to 400 N of preload was observed. It could be shown, that this is not a general stiffening effect in the case of a strong sigmoid shape of the hysteresis curve. Whenever the low-stiffness (LS) is considerably lower than the high-stiffness (HS), HS is not affected by preload, but LS is strongly increased (50– 80% in native case). With increasing rigidity of additional instrumentation the stiffening effect is reduced on LS and increased on HS. The stiffness-ratio (SR = HS/LS) is reduced to about 50% due to 400 N preload in the native case. If implants are evaluated by an instrumented-to-native (I/N) comparison, the effect of preload on the I/N ratio of the characteristic parameters is only minimal and specific to the instrumentation used.


    spine, preload, in-vitro testing, flexibility testing, six degree-of-freedom, loading device

    Author Information:

    Linke, B
    AO Research Institute, Davos,

    Meyer, G
    AO Research Institute, Davos,

    Knöller, S

    Schneider, E
    AO Research Institute, Davos,

    Committee/Subcommittee: F04.25

    DOI: 10.1520/STP11143S